1. Technical Field
The present invention relates to an ultrasonic flowmeter improved in its system for controlling the gain of the amplifier for amplifying the received signals.
2. Description of the Prior Art
The arrangement of an ultrasonic flowmeter of the prior art and the system for controlling the gain thereof will now be described with reference to FIG. 4.
A pair of ultrasonic transducers 2a and 2b are provided on both ends of a straight section 1a of a flow tube 1 including a fluid inlet 1b and a fluid outlet 1c extending vertically from the ends of the straight section 1a. 
A switching device 3 is provided between either of the ultrasonic transducers 2a and 2b and either of an excitation pulse generator 4 and an amplifier 5 receiving the signal from one of the transducers. The switching device 3 delivers pulses from the excitation pulse generator 4 to one of the transducers and receives signals from the other of the transducers and delivers the signals to the amplifier 5 and vice versa. The signal for causing the switching is not described herein.
The amplifier 5 has an input terminal 5a, an output terminal 5b, and a gain control terminal 5c for controlling the amplifying gain.
The data reduction equipment 6 of the prior art has a flow measuring section 6a in which the signals delivered from the amplifier 5 are processed to obtain the flow rate output. The flow rate measuring system such as a system of a transit-time difference type or a system of a phase difference type is known. No description thereon will be made herein.
The element denoted by the reference numeral 7 is adapted to be a peak detection circuit including, for example, a peak rectifier diode 7a and a smoothing capacitor 7b, etc.
The output of the peak detection circuit 7 is sent to gain controlling section 6b, and compared with a peak value preset in the controlling section 6b. On the basis of the result obtained, the voltage to be delivered to the gain control terminal 5c of the amplifier 5 is controlled to keep consistent a magnitude of the received signal.
However the following technical problems are still present in the system for controlling the gain of the prior art.
In the case that impurities such as bubbles are included in the measured fluid, the amplitude of the received signal tends to attenuate by the presence of the bubbles, so that the magnitude of the received signal is varied as shown in FIG. 5. In FIG. 5, the received waveform with no interference from bubbles is illustrated by a solid line, while the waveform attenuated by the effect of the bubbles is illustrated by a broken line. The peak values P included in the received waveform are decreased or eliminated with the passage of time as shown in FIG. 6.
In the gain control system of the prior art, the peak values are apt to be varied under the effect of the bubbles mixed with the fluid to be measured. In other words, the gain controlling signals are also varied under the effect of so called hunting phenomena, i.e. the waveform is saturated by excessive gain to make the received signal unstable.
While the gain is fixed in a certain value for avoiding the above-mentioned saturation of the waveform, sometimes the received signal is low depending on the species or the temperature of the fluid. In such case, the measurement should be made under the low S/N condition.
In the ultrasonic flowmeter of the prior art, systems are known for finding the flow rate from the received signals independently of the magnitude of the signals such as systems of the zero-cross type, the correlation type, or the phase differential type. These systems are advantageous when the received signals are tend to vary by the effect of bubbles included in the fluid. However, when the received waveform is unstable, it is difficult to measure the flow rate stably and precisely even if the above-mentioned systems are adopted.
In order to solve the above mentioned problem, the first ultrasonic flowmeter of the present invention comprises:
a pair of ultrasonic transducers mounted on an outer surface of a flow tube at an upstream side and a downstream side respectively, a switching device for switching the operational mode of each of the pair of transducers alternatively to its transmitting or receiving mode, an amplifier for amplifying the signal representing the ultrasonic waves propagating through the fluid received by the ultrasonic transducer of the receiving side, and a data reduction equipment for processing the amplified received signal to output a flow rate signal;
the amplifier includes an amplification gain control terminal, wherein an amplification gain is adapted to be adjusted on the basis of the control signal input into the terminal;
said data reduction equipment includes an analog-digital converter, a processor, and a memory, wherein converted into digital data are waveform data comprising analog data including a peak value of each of the received signals generated together with a plurality of transmission and reception, and the obtained digital data are stored in the memory;
the processor screens the peak values stored in the memory to obtain a screened peak value achieved on the fluid including no impurities such as bubbles or solid particles, compares the obtained peak value with the preset peak value set preliminary on the processor, sends a control signal to the gain control terminal of the amplifier on the basis of the result of the comparison, and controls the peak value included in the output signal of the amplifier so as to take the substantially equal value as that of the preset value of the peak.
According to the first embodiment, said screened peak value is the highest peak among the peaks included within the predetermined time interval.
According to the second embodiment, said screened peak value is the highest peak among the peaks included within the predetermined time interval, and values of the waveform data thereof will satisfy the requirements of the received waveform.
A second ultrasonic flowmeter of the present invention comprises:
a pair of ultrasonic transducers mounted on an outer surface of a flow tube at an upstream side and a downstream side respectively, a switching device for switching the operational mode of each of the pair of transducers alternatively to its transmitting or receiving mode, an amplifier for amplifying the signal representing the ultrasonic waves propagating through the fluid received by the ultrasonic transducer of the receiving side, and a data reduction equipment for processing the amplified received signal to output a flow rate signal;
the amplifier includes an amplification gain control terminal, wherein an amplification gain is adapted to be adjusted on the basis of the control signal input into the terminal;
said data reduction equipment includes an analog-digital converter, a processor, and a memory, wherein converted into digital data are waveform data comprising analog data including a peak value of each of the received signals generated together with a plurality of transmission and reception, and the obtained digital data are stored in the memory;
the processor calculates the amount of variation through comparing the peak value of the predetermined time interval with the peak value of the preceding time interval, and provided that the amount of variation does not exceed the predetermined threshold value, the peak value obtained in the present time interval is further compared with a preset peak value, and a control signal is sent to the gain control terminal of the amplifier on the basis of the result of the comparison, and the peak value included in the output signal of the amplifier is controlled so as to take the substantially equal value as that of the preset value of the peak, and on the other hand provided that the amount of variation does exceed the predetermined threshold value, the gain control operation of the amplifier is interrupted.
According to the third embodiment, the rates of increasing and decreasing of the gain are differentiated by the processor upon gain controlling operation of the amplifier.